In a device of the kind known from German Offenlegungsschrift No. 20 61 027 and German Patent Specification No. 20 65 048, the cooling channels, on the one hand, and the vacuum channels, on the other hand, are connected together in each of the relevant half shells. The cooling water, while flowing through each half shell, is under a substantially reduced pressure, which even leads to partial evaporation, whereby the cooling effect is enhanced. This known arrangement has basically proved extraordinarily satisfactory; however, the problem arises that condensate gets into the mold interior through the connections between the respective mold recess and the vacuum channel, and damages the surface of the pipe which is to be manufactured, from a thermoplastic flexible tube. A further problem which has arisen is that, in order to obtain an optimally finished type of pipe, temperatures which differ widely from one another, but which must be as far as possible constant for a particular process, have to be maintained in the half shells of the mold, and these depend firstly, on the pipe diameter and the pipe wall thickness, which have been selected, and secondly, on the composition of the plastic used, such as polyethylene, polypropylene or polyvinylchloride.
An apparatus is known from German Offenlegungsschrift No. 27 32 635 for the continuous production of plastic pipe having transverse profile features, which apparatus works according to the blow molding method rather than the so-called vacuum method. In this apparatus the half shells are guided within cooled guiding channels, through which the removal of heat also takes place.
In an apparatus known from German Offenlegungsschrift No. 25 37 184 for the manufacture of the above-mentioned plastic pipes, means are provided to cool the half shells, which means are attached to respective cooling water feed and outlet conduits circulating therewith.